NOx emissions are a product of combustion processes and contribute to major pollution problems such as acid rain. Two processes for de-nitrification are SNCR (Selective Non-Catalytic Reduction) and SCR (Selective Catalytic Reduction). Both processes commonly use ammonia as a reducing agent in the de-nitrification process of converting NOx into nitrogen and water vapor.
With the SNCR (Selective Non-Catalytic Reduction) process, ammonia is injected directly into combustion flame at temperatures that range from about 878 to 1158 degrees C. (1600 to 2100 degrees F.). The ammonia directly reacts with the NOx, reducing the emissions by 30–70%.
With the SCR (Selective Catalytic Reduction) process, ammonia vapor is injected into the flue gas stream at temperatures from 318 to 430 degrees C. (600 to 800 degrees F.). The gas then passes over a catalyst where the reaction occurs reducing the emissions by 80 to 90%. In this process, atomization and control of droplet size are critical to the reaction process because of the reduced reaction temperature necessitated by the operating temperature of the catalyst.
In both reduction methods an injection system for the ammonia is used. Because of the higher operating temperatures with the SNCR process, hydraulic nozzles can be used without the necessity for pressurized air atomization of the liquid reducing agent. In such SNCR processes, hydraulic nozzles are mounted on lances that extend into the combustion flame. In the SCR process, air-atomizing nozzles are mounted on lances that extend into the discharging gas stream. Because of the lower temperatures at such location, the injection device must supply small droplets that vaporize quickly.
Safety concerns with anhydrous and aqueous ammonia has increased interest in using urea as a safe and economical alternative. The major problem with urea is that it is temperature sensitive. The temperature of the urea must be maintained below 70° C. (158° F.) prior to atomization and direction to avoid crystallization. If the urea crystallizes due to prior exposure to high temperatures it will clog the injection piping and discharge orifices. Atomization and control of droplet size also are critical to the reaction process because any crystallization of the urea prior to atomization and discharge is detrimental to reaction process.